Economical purification of bisphenol a

ABSTRACT

Process and related apparatus are disclosed for economically producing a very high quality bisphenol A product by a multistage crystallization process utilizing a new cross-flow wash design whereby a high purity wash phenol stream is fed to every washing stage, optionally also in combination with a modified stream flow/recycle design which avoids increased by product production and yield losses otherwise expected from processing increased volumes of wash phenol.

FIELD OF THE INVENTION

[0001] P, P Bisphenol A (BPA) is a commercially significant compoundused to manufacture polycarbonates and epoxy resins. The polycarbonateapplication in particular demands high purity BPA due to stringentrequirements for optical clarity and color in the finished application.Accordingly those skilled in the art continually strive to improve theproduct quality of bisphenol A in economically efficient processschemes. This invention concerns a new process scheme to purifybisphenol A via adduct crystallization.

BACKGROUND OF THE INVENTION

[0002] To understand this invention it is first necessary to understandthe prior art for BPA, which includes U.S. Pat. No. 4,950,805 (Iimuro,et al.) and U.S. Pat. No. 5,345,000 (Moriya et al.), which patents areincorporated herein by reference.

[0003] BPA is formed by the condensation of two moles of phenol with onemole of acetone under acidic conditions. The reaction may take place inthe presence of a strong homogenous acid, such as hydrochloric acid,sulfur acid, or toluene sulfonic acid, or in the presence of aheterogeneous acid catalyst, such as a sulfonated ion exchange resin.The reaction may take place in the absence or presence of a thiolpromoter, such as a mercaptan, which may be homogeneous in the reactionmixture or fixed to a heterogeneous catalyst A stoichiometric excess ofphenol is employed to improve selectivity to the highly desired p,p′-BPAisomer. The crude reaction effluent stream thus contains not onlyunconsumed phenol, p,p′-BPA and water byproduct from the condensationreaction, but also contains undesirable BPA isomers and impurities, suchas the o,p-BPA isomer, trisphenols, codimers, spiroindanes, and coloredimpurities as well as unreacted acetone, homogeneous acid catalysts, andthiol promoters. Those skilled in the art of producing BPA, recognizethat regardless of the method of reacting acetone and phenol to produceBPA, it is necessary to purify the crude reaction effluent stream toobtain a BPA product that is useful for the manufacture ofpolycarbonates and other engineering thermoplastics.

[0004] A conventional practice in the purification of bisphenol A can begenerically described by reference to FIG. 1.

[0005] Acetone feed to reaction (1) is either makeup acetone (21) or acombination of makeup acetone and unreacted recycle acetone (23)recovered from the downstream distillation system (2). Phenol feed tothe reaction consists of distilled phenol (22) from the downstreamdistillation section (2) or phenol recycle (24) produced as the liquidseparation or wash product from the downstream solid/liquid separation(4), and typically contains a large quantity of impurities and BPA, or acombination of these two streams. Stream (22) may in some cases becombined with the acetone recycle stream 23. The reaction systemeffluent (25) is sent to a distillation system (2), which recoversunreacted acetone (23), phenol (22) and waste streams, such as thewastewater of condensation (40). These lighter fractions are separatedfrom the reactor effluent stream to form a concentrated BPA stream (26),which still contains a large quantity of phenol.

[0006] In the adduct crystallization technology that is widelypracticed, for example in U.S. Pat. No. 4,927,978, U.S. Pat. No.4,950,805 and U.S. Pat. No. 5,345,000, sufficient cooling of theconcentrated BPA and phenol stream produces a crystalline adduct whichcomprises one molecule of phenol and one molecule of the desiredp,p′-BPA isomer. This crystallization is a relatively efficient way topurify p,p′-BPA from the other isomers and impurities contained in thestream. The crystallization, depending on purity requirements andeconomic considerations, can be conducted in a selected number ofmultiple stages, with each stage producing a much higher purity adductcrystal. Each crystallization stage may consist of a single crystallizeror a multitude of crystallizers in series. The preferred number ofcrystallizers per stage is between 1 and 3. The number ofcrystallization stages may be any number greater than or equal to two.For illustration purposes in FIG. 1, three stages of crystallization (3,5 and 7) are indicated. The effluent (27) from the first crystallizationstage (3) is sent to a solid-liquid separation device (4). These devicescan use hardware well known to those skilled in the art, such as filtersor centrifuges. A key feature in all of these systems is to firstseparate the bulk of the liquid to produce a solids crystal cake and amother liquor (filtrate/centrate) stream, and then to wash the cake toremove impurities. Phenol is typically used to effect this wash, and theultimate product quality is strongly determined by the purity of thewashing agent. Phenol is a useful washing agent to the extent that itdoes not introduce new impurities into the BPA-phenol adduct crystal.

[0007] The solid-liquid separation devices (4, 6 and 8) produce anadduct stream (respectively 35, 36, 37) and one or more liquid streamscontaining some mixture of spent wash and/or filtrate/centrate (e.g.,28, 33, 34, 54, 53). For the purposes of adduct dissolution, crystalrecovery, and for the enhancement of crystallization and material flow,those skilled in the art will typically recycle some or all of each ofthe mother liquor and wash effluent streams from one separation stageback to the preceding crystallization stage (e.g., 33, 34).

[0008] The adduct produced from each stage may be placed in a phenolsolution again and recrystallized to afford additional purification. Thefinal adduct (37) is sent to the BPA finishing apparatus (9), wherein aliquid state phenol is vaporized in some fashion away from BPA toproduce a high purity molten BPA product (51). The clean phenol (38) soproduced can be combined with the makeup phenol to the plant (50) toprovide a solvent (52) for washing the solid adduct crystals producedin, at least, the last solid-liquid separation (8).

[0009] It is known to those skilled in the art that an efficient meansto produce a high purity product through multi-stage crystallization isto send the cleanest wash liquid to the final stage, and in turn sendsome combination of filtrate/centrate or spent wash from each stage aswash (53, 54) to the immediate upstream stage. Such a scheme effectivelyminimizes the phenol requirements for the overall crystallization washsystem, and uses the cleanest phenol where it is most necessary, i.e.,in the final stage. This wash scheme is usually referred to as acounter-current wash flow.

[0010] The total liquid (28) recovered from the first stage solid/liquidseparation (4), a combination of mother liquor and wash effluent, isrecirculated to the condensation reactor system (1). Optionally a stream(29), consisting of all or a portion of the recycle liquid (28), may besent to a recovery system (10) which rearranges unwanted heavybyproducts contained in the recycle liquid to useful precursors of BPAor to BPA itself A waste stream (32) is removed, and the improved stream(30) is returned to the recycle liquid. Some of the recycle liquid (31)may bypass the recovery system (10). The combined streams (30) and (31)are used as a reactor feed stream (24).

SUMMARY OF THE INVENTION

[0011] By contrast, in the present invention, as illustrated in FIG. 2,the washing scheme is changed in order to provide an even higher purityproduct, and to accomplish this beneficial result without compromisingthe yields of raw materials to product. In addition, other processmodifications shown in FIG. 2 and described below cooperatesynergistically with the modified wash scheme to achieve unexpectedlyenhanced results.

[0012] In accordance with the present invention, it has now been foundthat an extremely high quality BPA product can be produced by co-feedinga clean or substantially clean phenol stream to every stage of washing.This type of washing is herein referred to as a cross-flow wash system.While generally counter-current wash flow maximizes product purity for agiven quantity of wash supply, in some processes, particularlycrystallization, where impurities are bound in the crystal product ateach stage, and, therefore, cannot be washed out, we have found thatincreased product purity can be obtained by using a cross-flow washsystem in accordance with this invention as long as an increased washsupply can readily be made available. With the prior art BPA processes,however, wash flows were limited to the amount of fresh and recoveredphenol (e.g., 50, 38 in FIG. 1) because the use of additional phenolfrom other sources within the plant would degrade plant yields. Higherquantities of wash in the conventional counter-current flow schemescreate increased flow of recycle liquor back to the condensationreactor, and the raw material consumption of the overall processworsens. Moreover, the amount of BPA lost in the purge (32) alsoincreases due to a combination of a higher make of impurities in thereactor as well as a more dilute mother liquor stream with respect toimpurities but not p,p′-BPA For these reasons, no one of ordinary skillin this art based on existing knowledge and experience with conventionalBPA processes would be led to try to employ higher quantities of wash inconjunction with counter-current flow.

[0013] In the present invention, however, a new approach is disclosedfor integrating the process operations to provide a purer productwithout suffering from expected yield losses. This invention isdescribed by reference to FIG. 2. FIG. 2 shows the utilization of thesame overall unit operations as seen in FIG. 1, where the same orcomparable elements are indicated by the same reference numbers, but theintegration between these blocks is substantially different Thefollowing description points out the key differences between this scheme(FIG. 2) and the prior art (FIG. 1).

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a representative process flow diagram of BPA productionaccording to one common conventional technology.

[0015]FIG. 2 is an illustrative process flow diagram of a preferredembodiment of a BPA process in accordance with the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0016] As shown in FIG. 2, in accordance with the present invention thewash (54, 53, 52) supplied to the staged solids/liquid separationsystems (blocks 4, 6 and 8) now flows in a cross-flow scheme. As aresult of this design modification, the wash to each stage now containsonly a high purity phenol, with high purity composition being fed toeach stage. The high purity phenol is supplied to blocks 4, 6 and 8 viastreams 54, 53 and 52 respectively. Some of this high purity phenol (82)is supplied from a combination of fresh makeup phenol to the plant(stream 50) and phenol recovered from the high purity adduct in thefinishing system 9 (stream 38). In order to obtain sufficient quantitiesof such high purity phenol to maintain adequate wash rates for themultiple solids/liquid separation devices, the BPA process of thepresent invention introduces a new source of high purity phenol, namelystream 81, which is recovered from the distillation section (2).Alternatively, utility costs can be reduced by producing a slightly lesspure phenol in the distillation section (2), and feeding his wash (81)only to the upstream crystallization stages (4 and 6 in FIG. 2), whilefeeding the fresh and recovered phenol (82) of somewhat higher purityonly to the last downstream crystallization stage (8 in FIG. 2).

[0017] By material balance, the net quantity of recycle liquid (28)leaving the first separation stage (4 in FIG. 2) must be larger undersuch a scheme than with the process shown in FIG. 1. This higher phenolflow, which will be saturated with BPA, results in more BPA content inthe feed to the condensation reaction system 1, as all the flow isreturned to the reactor system, leading to increased byproduct make inthe reactors which ordinarily would be expected to result in a loss inyield.

[0018] In this invention, however, it is proposed to avoid this highrecycle phenol flow. The additional high purity wash phenol (81) will begenerated in the distillation section (2) by removing phenol from thecrystallizer feed stream (26). In the prior art BPA process, thismodification would have expected to lead to problems in thecrystallizer, since phenol and BPA concentrations in the feed stream(26) are optimized for crystallizer performance. In this invention, bycontrast, it has been found that these problems can be completely orsubstantially avoided by immediately diluting stream 26 with a phenolflow (70) obtained selectively from the wash effluent and mother liquorflows from the crystallization stages. This step of the presentinvention is therefore another key distinction from the prior art BPAprocesses. By material balance it can be shown that when the flow of thedilution stream (70) substantially equals the flow of the new highpurity phenol supply (81), there is substantially no increase in therecycle liquid (28) returning to the condensation reactors from theseparation section.

[0019] Furthermore, the present invention may also include the step ofcreating the dilution stream (70) by selectively taking, as needed, partor all of the wash effluent and mother liquor streams of the separationstages (4, 6, 8) starting with the last stage, to create a streamcontaining the least amount of impurities. In a preferred embodiment ofthe present invention, the purity of the final product is enhanced ifstream 70 is drawn so as to contain a minimum of impurities. Therefore,it will preferentially be made up of, in order of preference, flow fromthe wash effluent, then from the mother liquor of the last separationstage, then by adding flows from the wash effluent, and then from themother liquor of each preceding separation stage, until the desiredvolume of flow is achieved.

[0020] It will be apparent to those skilled in the art that otherchanges and modifications may be made in the above-described apparatusand methods for economical purification of bisphenol A without departingfrom the scope of the invention described herein, and it is intendedthat all matter contained in the above description shall be interpretedin an illustrative and not a limiting sense.

What is claimed is:
 1. In a process for the purification of bisphenol Avia crystallization of phenol adduct from a phenol solution in amultistage process, the improvements comprising the steps of a.producing phenol adduct crystals in each of a plurality of purificationstages; and, b. washing the phenol adduct crystals produced in each ofsaid purification stages with a high purity wash phenol stream that issubstantially free of bisphenol isomers and impurities.
 2. A processaccording to claim 1 further comprising the step of dewatering thephenol adduct crystals from each stage before washing them with saidhigh purity phenol stream.
 3. A process according to claim 1 furtherwherein the liquid feed stream to the first upstream purification stagecomprises about 30-60% by weight of p,p′-BPA.
 4. A process according toclaim 1 further wherein the liquid feed stream to the first upstreampurification stage comprises about 30-55% by weight of p,p′-BPA.
 5. Aprocess according to claim 1 further comprising the step of recycling atleast a first portion of spent wash or separated mother liquor from oneor more of said purification stages for use as a dilution solvent toreduce the concentration of p,p′-BPA in the mixed crystalliation feed tothe first upstream purification stage to below 40% by weight.
 6. Aprocess according to claim 1 further comprising the step of recycling atleast a first portion of spent wash or separated mother liquor from oneor more of said purification stages for use as a dilution solvent toreduce the concentration of p,p′-BPA in the mixed crystallization feedto the first upstream purification stage to about 25-40% by weight.
 7. Aprocess according to claim 5 further comprising the step of recycling atleast a second portion of said spent wash or separated mother liquor tothe crystallizers in the first purification stage.
 8. A processaccording to claim 7 further comprising the step of recycling remainingspent wash and separated mother liquor to a reaction system forproduction of p,p′-BPA.
 9. A process according to claim 8 wherein saidp,p′-BPA is produced by condensation of acetone with phenol or from thereaction of heavy byproducts.
 10. A process according to claim 1 whereinsaid high purity wash phenol stream includes fresh makeup phenol to theprocess.
 11. A process according to claim 1 wherein said high puritywash phenol stream includes phenol produced in a downstream step ofdevolatilizing BPA-phenol adduct recovered from the final solid/liquidseparation step to produce phenol and a molten BPA product stream.
 12. Aprocess according to claim 1 wherein said high purity wash phenol streamis produced by volatilization of phenol from the BPA process.
 13. Aprocess according to claim 12 further wherein said phenol-containingprocess stream originates from a condensation reactor.
 14. A processaccording to claim 7 wherein a predominant proportion of said spent washis preferentially recycled to the first crystallization stage.
 15. Aprocess according to claim 1 wherein spent wash from downstreamcrystallization stages is recycled to the first crystallization stage.16. In a process for the purification of bisphenol A via crystallizationof phenol adduct from a phenol solution in a multistage process, theimprovements comprising the steps of: a producing phenol adduct crystalsin each of a plurality of purification stages; and, b. washing thephenol adduct crystals produced in each of said purification stages witha high purity wash phenol stream that is substantially free of bisphenolisomers and impurities, wherein phenol adduct crystals produced in atleast the final downstream purification stage, and, optionally, in oneor more other downstream purification stages are washed with freshmakeup phenol fed to the process, or with phenol produced from afinishing step for obtaining a final BPA adduct product, or a mixturethereof, and further wherein phenol adduct crystals produced in thefirst upstream purification stage, and, optionally, in one or more otherupstream purification stages are washed with phenol produced fromphenol-containing effluents produced in the BPA process.
 17. A processaccording to claim 16 further comprising the step of dewatering thephenol adduct crystals from each stage before washing them with saidhigh purity phenol stream.
 18. A process according to claim 16 furtherwherein at least a portion of separated mother liquor or spent washproduced from the multistage crystallization is recycled immediatelyupstream of or to the first stage adduct crystallization stage.
 19. Aprocess according to claim 18 further wherein remaining separated motherliquor and spent wash are recycled to a reaction system for theproduction of p,p′-BPA either from the condensation of acetone withphenol or from the reaction of heavy byproducts.
 20. A process accordingto claim 16 further wherein mother liquor and spent wash produced fromthe downstream crystallization stages are preferentially recycled to thefirst crystallization stage.
 21. A process according to claim 20 furtherwherein spent wash is recycled preferentially to the mother liquor. 22.A process for the purification of bisphenol A via adduct crystallizationof phenol adduct from a phenol solution in a multistage process whereinthe adduct crystals are separated from the mother liquor, dewatered, andwashed with a phenol stream substantially free of bisphenol isomers andimpurities.
 23. A process according to claim 22 wherein the phenolstream for washing includes makeup phenol to the process.
 24. A processaccording to claim 22 wherein the phenol stream for washing includesphenol produced from the finishing of BPA-phenol adduct to produce BPA.25. A process according to claim 22 wherein the phenol stream forwashing includes phenol produced from phenol-containing process streamswithin the BPA process.
 26. A process for the purification a bisphenol Avia crystallization of phenol adduct from a phenol solution in anmultistage process wherein the adduct crystals of each purificationstage are separated from the mother liquor, dewatered, and washed with aphenol stream substantially free of bisphenol isomers and impurities,the downstream stages being preferentially washed with makeup phenol tothe process or phenol produced from the finishing of phenol-BPA adduct,and the upstream stages being washed with a slightly less pure phenolproduced from phenol-containing process streams within the BPA process.